In general, failure rate is the frequency in which a component fails (e.g., the number of failures per hour). Reliability theory often denotes this rate using the Greek letter λ (lambda).
An example of a failure rate metric used by circuit board designers and manufacturers is the Mean Time To Failure (MTTF) which is simply the inverse of the failure rate (i.e., 1/λ). Accordingly, MTTF for a circuit board can be expressed as the average number of circuit board operation hours per failure.
Another example of a failure rate metric used by circuit board designers and manufacturers is the Mean Time To Repair (MTTR). MTTR for a circuit board can be expressed as the average number of hours required to repair a failed circuit board.
Yet another example of a failure rate metric used by circuit board designers and manufacturers is the Failures In Time (FIT) rate which is the number of failures that can be expected in one billion (109) hours of circuit board operation. Circuit board FIT rates are often expressed in terms of one billion hours since circuit board failure rates may be extremely low.
In connection with circuit board design, many circuit board designers maintain internal databases of failure data on circuit boards that the designers ultimately produce. The circuit boards designers then use this historical data to calculate failure rates for those circuit boards. Additionally, the circuit boards designers may calculate failure rates for future circuit boards according this historical data since the historical data may further serve as a useful estimate.
Alternatively, circuit boards designers may test samples of actual manufactured circuit boards in order to generate failure rate information. However, such circuit board testing may be prohibitively expensive or impractical. For example, to accumulate actual information in a test laboratory, such testing may require (i) sacrificing an unreasonable number of very expensive circuit boards and/or (ii) monitoring the circuit boards for an excessive amount of time.